[ci skip] document default sites in nginx. closes #650

docs/nginx.md

@@ -158,3 +158,18 @@ CONTAINER ID        IMAGE                      COMMAND                CREATED
 d6499edb0edb        dokku/node-js-app:latest   "/bin/bash -c '/star   About a minute ago   Up About a minute   0.0.0.0:49153->5000/tcp   nostalgic_tesla
 
 ```
+
+# Default site
+
+By default, dokku will route any received request with an unknown HOST header value to the lexicographically first site in the nginx config stack. If this is not the desired behavior, you may want to add the following configuration to nginx. This will catch all unknown HOST header values and return a `410 Gone` response.
+
+```
+server {
+  listen 80 default_server;
+  listen [::]:80 default_server;
+
+  server_name _;
+  return 410;
+  log_not_found off;
+}
+```